1. Field of the Invention
This invention relates to an annealing furnace for coiled steel plates, and more particularly to a rotary hearth finish annealing furnace for finish annealing coiled steel plates such as anisotropic electromagnetic steel plates or the like coated with an annealing parting agent.
2. Description of the Prior Art
The anisotropic electromagnetic steel plate is produced in steps of at least one time annealing and cooling hot rolled steel plates conditioned to contain less than 0.085% of carbon, less than 4% of silicon and less than 0.07% of elements aiding in secondary recrystallization such as sulfur, aluminum or the like, continuously decarburizing annealing the plates, coating the plates with a slurry of annealing parting agent such as magnesia, drying the coated plates, winding the dried plates into a coil, and finish annealing the coil. The finish annealing is usually effected by immersing the coil in a high pure reducing atmosphere gas at a high temperature (higher than 1,100.degree. C.) for a long time (more than 10 hours) and then cooling to lower than 450.degree. C., for the purpose of producing the secondary recrystallization and surface films and removing impurities.
The finish annealing has been effected in an annealing furnace as shown in FIG. 1, a schematic plan view of the furnace and FIG. 2, a sectional view taken along the line II--II in FIG. 1. A hearth or furnace bed 1 is supported by rollers 2 so as to travel along a circular line having a predetermined radius (for example 12.5 m). A substantially half of the circular hearth 1 is covered by a heat retaining cover 4 having burners 3 to form a heating zone 5 and one fourth next thereto is covered by a heat retaining cover without heating means such as burners to form a cooling zone 6 in the furnace. A further bed portion contiguous to the cooling zone 6 is not covered to form an external cooling zone 7 whose terminal end is provided between it and the heating zone 5 with a loading and unloading zone 8. Coil tables 10 are provided on the bed 1 through supports 11 so as to be able to locate steel plate coils 9 whose axes are vertical. The burners 3 are located at a level somewhat higher than an upper surface of the hearth 1. The steel plate coils 9 are covered by inner covers 12 filled with a reducing atmosphere gas such as hydrogen and are heated by burning gas from the burners 3 and the reducing atmosphere gas heated and raised in temperature by the burners 3. A reference numeral 13 in FIG. 2 denotes sealing means for keeping the hearth 1 and the heat retaining cover 4 in an air-tight manner.
With the above annealing furnace, however, the steel plate coils 9 are arranged in a single circular row without arranging them side by side in radial directions of the circular hearth and without piling them one upon the other as shown in phantom lines in FIG. 1. Its productivity is not necessarily high and its heat radiating area of the furnace wall per one coil is unduly large to consume a comparatively great amount of energy for the operation of the annealing furnace.
In order to avoid such disadvantages, it can be proposed to arrange two coil tables 10 and 10a one above the other to heat the coils 9 in an upper and a lower circular row. With such an arrangement, the productivity is improved and the heat radiating area per one coil is small. However, support members 14 for supporting the upper coil table 10a are needed and therefore to increase the heat capacity as a whole and to make difficult handling the coils 9 for loading and unloading the coils on and from such a high level.
In the above annealing furnaces shown in FIGS. 1-3, the burners 3 are located at the lower level of the furnaces to cause the burning gas and the heated gas raised in temperature having light specific weight to rise so as to heat the upper and intermediate portions of the furnaces with the aid of these gases, thereby maintaining the temperature in the furnaces substantially constant as a whole. However, because of the burners 3 located at the relatively low positions, the hearth 1 is heated directly by the burners 3, so that the temperature of the hearth 1 becomes high substantially to the degree of the ceiling of the furnace. As the result, the difference in temperature between the hearth and the atmosphere becomes great to increase the quantity of heat transmitted from the hearth to the atmosphere, i.e. dissipating heat radiated from the hearth. In addition, the heat accumulated in the hearth 1 increases and therefore to reduce the thermal efficiency with resulting high running cost.
As above described, the movable hearth 1 are subjected to heating and cooling cycles over a wide temperature range, consuming the thermal energy for raising the temperature of the movable hearth 1.
FIG. 4 illustrates a movable hearth as an example of the prior art. The movable hearth 1 supports thereon coil tables 10, coils 9 to be annealed and inner covers 12. A load acting upon the supports 11 is supported by firebricks 1a pyramidally piled in the hearth 1. A weight of the inner cover 12 is supported by heat insulating bricks 1b piled about the firebricks 1a, about which refractory casters 1c are provided. A support metal members 1d are provided on the sides of the heat insulating bricks 1b to support traverse thermal expansion of the heat insulating bricks 1b. The movable hearth 1 is accommodated in its entirety in hearth metal members 1e. A sealing material 1f, for example, mullite sand or the like is filled between the lower end of the inner cover 12 and the heat insulating bricks 1b for sealing atmosphere gas in the inner cover 12.
In this manner, the movable hearth 1 of the prior art comprises the firebricks 1a having a bulk specific gravity of 2.0 and a thermal conductivity of 2.4 Kcal/mh.degree.C. (at 1,000.degree. C.), the heat insulating bricks 1b having a bulk specific gravity of 0.7 and a thermal conductivity of 0.55 Kcal/mh.degree.C. (at 1,000.degree. C.) and the refractory casters 1c having a bulk specific gravity of 1.5 and a thermal conductivity of 1.4 Kcal/mh.degree.C. (at 1,000.degree. C.). The movable hearth thus constructed is rigid, but heavy and has a thermal conductivity of more than 0.8 Kcal/mh.degree.C. (at 1,000.degree. C.) as a whole resulting in a great heat loss.
When the movable hearth 1 expands at a high temperature, moreover, the sealing material 1f penetrates into joints of the heat insulating bricks 1b. As the result, when cooled, the bricks 1b are subjected to forces to be expanded in traverse directions of the hearth, so that the bricks 1b progressively move away from each other resulting finally in damage to the movable hearth.